1. Field of the Invention
The present invention relates to a stencil printing machine which conveys a print medium while pressing the print medium to a drum on which a stencil sheet is mounted, and transfers an ink oozing from perforations of the stencil sheet onto the print medium.
2. Description of the Related Art
As a conventional printing method of a stencil printing machine, there are an inner press method (refer to Japanese Patent Laid-Open Publication No. Hei 7-132675 (published in 1995)) and an outer press method (refer to Japanese Patent Laid-Open Publication No. 2001-246828).
The inner press method is briefly described. As shown in FIG. 1, a drum 100 and a back press roller 101 are provided, and the drum 100 and the back press roller 101 are provided so as to be freely rotatable individually in a state where outer peripheral surfaces thereof are partially made substantially adjacent to each other. A stencil clamping portion 100a which clamps a tip end of a stencil sheet 104 is provided on the outer peripheral surface of the drum 100, and an outer peripheral wall other than the stencil clamping portion 100a is formed of a screen 102 which is flexible and ink permeable.
An ink supply mechanism 105 is provided inside the drum 100. As shown in FIG. 2, this ink supply mechanism 105 includes an inner press roller 106 which is an ink supply roller, and the inner press roller 106 is provided on a roller support member 107 so as to be freely rotatable. The inner press roller 106 is configured to be shiftable between a press position where the roller support member 107 is energized in a direction of an arrow a of FIG. 2 to press an inner peripheral surface of the screen 102 and a standby position where the roller support member 107 is rotated in a direction of an arrow b of FIG. 2 to be spaced from the inner peripheral surface of the screen 102. The inner press roller 106 is set at the press position when a print sheet 111 passes therethrough, and otherwise, set at the standby position. Moreover, the inner press roller 106 has a function to apply printing pressure from an inner periphery side of the screen 102.
The roller support member 107 is supported so as to be freely rotatable about a support shaft 108, and a doctor roller 109 and a drive rod 110 are individually provided on the roller support member 107. The doctor roller 109 has a cylindrical shape, and is fixed to the roller support member 107 at a position close to the inner press roller 106. The drive rod 110 is supported on the roller support member 107 so as to be freely rotatable, and is placed in an upper space composed of outer peripheral surfaces of the inner press roller 106 and the doctor roller 109 on sides thereof adjacent to each other. An ink 103 is supplied from an ink supply portion (not shown) to the upper space.
Next, printing operations are schematically described in order. The stencil sheet 104 on which a perforated image is formed is attached onto an outer peripheral surface of the screen 102. Then, during a printing mode, the drum 100 and the back press roller 101 are rotated in synchronization with each other in directions shown in arrows in FIG. 1, and the print sheet 111 is fed between the drum 100 and the back press roller 101.
When the print sheet 111 is fed, the inner press roller 106 presses the screen 102, and the inner press roller 106 rotates following the drum 100 in such a pressing state. The ink 103 having passed through a gap between the inner press roller 106 and the doctor roller 109 is adhered onto the outer peripheral surface of the inner press roller 106, and the ink 103 thus adhered is sequentially supplied to an inner surface of the screen 102 by the rotation of the inner press roller 106.
Moreover, when the inner press roller 106 presses the screen 102, the screen 102 swells out to the outer periphery side thereof by pressing force at this time, and the screen 102 is put into a press-contact state with the back press roller 101. Then, the print sheet 111 conveyed between the drum 100 and the back press roller 101 is conveyed while being brought into press contact with the screen 102 and the stencil sheet 104 in between the inner press roller 106 and the back press roller 101. By press-contact force at this time, the ink 103 on the screen 102 side is transferred to the print sheet 111 side from perforations of the stencil sheet 104, and an ink image is printed on the print sheet 111.
The outer press method is briefly described. As shown in FIG. 3, a drum 120 is provided. A stencil clamping portion 120a which clamps the tip end of the stencil sheet 104 is provided on an outer peripheral surface of this drum 120, and an outer peripheral wall 120b other than the stencil clamping portion 120a is formed of an ink permeable member with a porous structure.
An ink supply mechanism 125 is provided inside the drum 120. The ink supply mechanism 125 includes a squeegee roller 126 supported so as to be freely rotatable, and a doctor roller 127 placed adjacent to the squeegee roller 126. An ink 128 accumulates in an outer peripheral space surrounded by the squeegee roller 126 and the doctor roller 127. The ink 128 adhered onto the outer periphery of the rotating squeegee roller 126 passes through a gap between the squeegee roller 126 and the doctor roller 127, and thus only the ink 128 with a predetermined film thickness is adhered onto the squeegee roller 126, and the ink 128 with the predetermined film thickness is supplied to an inner surface of the outer peripheral wall 120b. 
Moreover, a pressure roller 130 is provided at a position opposite to the squeegee roller 126, which is also an outside position of the drum 120. The pressure roller 130 is configured to be shiftable between a press position of pressing the outer peripheral wall 120b of the drum 120 and a standby position of being spaced from the outer peripheral wall 120b of the drum 120. The squeegee roller 126 is fixed to a support member which supports the outer peripheral wall 120b of the drum 120 so as to be freely rotatable, and an outer peripheral surface of the squeegee roller 126 and the inner peripheral surface of the outer peripheral wall 120b of the drum 120 are brought into a state of being slightly spaced from each other in a state where the outer peripheral wall 120b of the drum 120 is not pressed by the pressure roller 130. When the outer peripheral wall 120b of the drum 120 is pressed by the pressure roller 130, the outer peripheral wall 120b of the drum 120 is bent, and thus the outer peripheral surface of the squeegee roller 126 and the inner peripheral surface of the outer peripheral wall 120b of the drum 120 are brought into contact with each other.
Next, printing operations are schematically described in order. The stencil sheet 104 on which the perforated image is formed is attached onto an outer peripheral surface of the outer peripheral wall 120b of the drum 120. Then, during the printing mode, the outer peripheral wall 120b of the drum 120 is rotated in a direction shown by an arrow in FIG. 3, and the print sheet 111 is fed between the drum 120 and the pressure roller 130.
When the print sheet 111 is fed, the pressure roller 130 presses the outer peripheral wall 120b of the drum 120, and the outer peripheral wall 120b is shifted toward an inner periphery side thereof. The outer peripheral wall 120b is brought into a pressed state on the squeegee roller 126 by such shifting, and the squeegee roller 126 rotates following the drum 120. Onto the outer peripheral surface of the squeegee roller 126, the ink 128 having passed through the gap between the squeegee roller 126 and the doctor roller 127 is adhered. The ink 128 thus adhered is sequentially supplied to an inner surface of the outer peripheral wall 120b by the rotation of the squeegee roller 126.
Moreover, when the pressure roller 130 presses the outer peripheral wall 120b of the drum 120, the print sheet 111 conveyed between the drum 120 and the pressure roller 130 is conveyed while being brought into press contact with the stencil sheet 104 in between the squeegee roller 126 and the pressure roller 130. By press-contact force at this time, the ink 128 on the outer peripheral wall 120b side is transferred to the print sheet 111 side from the perforations of the stencil sheet 104, and an ink image is printed on the print sheet 111.
Incidentally, in the stencil printing machines of the conventional inner press method and outer press method, ink pools are individually formed in the outer peripheral space of the inner press roller 106 and the doctor roller 109 and in the outer peripheral space of the squeegee roller 126 and the doctor roller 127, and the inks 103 and 128 in the ink pools are supplied to the screen 102 and outer peripheral wall 120b of the drums 100 and 120 at the time of printing. Hence, when the printing is not performed for a long time, the inks 103 and 128 having accumulated in the ink pools and the inks 103 and 128 adhered onto the drums 100 and 120 and the like are left standing in a state of being in contact with the atmosphere, and there is a problem that the inks 103 and 128 are degraded.